A procedure to infect lettuce seedlings with oospores of Bremia lactucae

Samenvatting Oösporen vanBremia lactucae kunnen worden verkregen door bladschijfjes van sla te inoculeren met een gemengde conidiënsuspensie van twee fysio's van de schimmel, die de twee compatibiliteitstypen (B₁ en B₂) vertegenwoordigen. Wanneer het bladweefsel geheel vergaan is, worden de schijfjes met oösporen samen met het onderliggende filtreerpapier op een dunne laag grond in een glazen schaaltje gebracht. Hierop worden vijf zaden van een vatbaar slaras te kiemen gelegd. Zaailingen die worden aangetast vertonen na drie tot vijf weken sporulatie op de cotylen. De conidiën worden op een toetssortiment van slarassen gespoten om het aantastingspatroon van het isolaat te bepalen. De eerste resultaten toonden aan dat de nakomelingen van een kruising gewoonlijk een beperkter aantastingspatroon hebben dan de ouders.     

Bremia lactucae populations on cultivated lettuce originate from prickly lettuce and are interconnected with the wild pathosystem

European Journal of Plant Pathology - Maize rough dwarf disease (MRDD) is one of the most important viral diseases around the world. Rice black-streaked dwarf virus (RBSDV) and maize rough dwarf...     

Inheritance of virulence in Bremia lactucae to match several resistance factors in lettuce

A 1∶1 mixture of spores of the physiologic races NL5 and NL6 ofBremia lactucae was sprayed on leaf discs of the lettuce cultivar Olof. The resulting oospores were allowed to germinate on seedlings after a two-month ripening period. Monospore lines of 49 of the 79 recovered isolates were tested on cotyledons and leaf discs of a differential series for virulence to 11 resistance factors. All monospore lines were also tested for the mating type factor. A backcross of a progeny line, line H, with NL5 was made and 41 isolates tested on the differential series. Results indicated complex inheritance based on more than one locus each for virulence to R2 and R11. Vilulence to match the resistance factor R4 seems to be based on a dominant factor or on a combination of a dominant and a recessive factor. Virulence to factors R5, R8 and R10 are recessive and possibly interchangeable. Inheritance of virulence to factors R3, R6 and R9 must be studied further before definite conclusions can be drawn. Virulence to R7 resulted in many incomplete reactions, but all showed significantly less sporulation than the virulent parent. Variations in necrosis in intermediate reactions indicate the presence of minor genes affecting these symptoms. Many of the avirulent loci in the parent lines proved to be heterozygous. Mating type segregation fits a 1∶1 ratio, indicating that this factor is controlled by a single gene. Selfing, mutation and somatic crossing-over can affect the ratios of recovered progeny and so our conclusions concerning the inheritance of virulence.     

Further characterization of field resistance in lettuce to Bremia lactucae (downy mildew)

The development of five Bremia lactucae isolates on the field-resistant crisphead lettuce cultivar Iceberg was compared under field conditions with that on the more susceptible crisphead cultivar Ithaca. With each of five isolates, the epidemic developed more rapidly on Ithaca than Iceberg; lesions on Ithaca were more numerous, larger and sporulated more profusely than on Iceberg. There was no evidence for adaptation of isolates to Iceberg, which provides some additional evidence that the resistance of this cultivar could be non-differential (horizontal). There was, however, evidence of non-specific differences between isolates in their quantitative virulence.     

The genetic control and expression of specificity in Bremia lactucae (lettuce downy mildew)

The inheritance of specific virulence in Bremia lactucae was studied in crosses involving 12 heterothallic isolates of the fungus. In one cross, virulence to eight of the II specific resistance factors examined segregated in the F₁ generation. Although there were exceptions, most of the data were consistent with the hypothesis that pathogenicity was controlled by independent single loci with avirulence dominant to virulence. Linkage between loci determining virulence on R2 and R11 was confirmed. Loci controlling virulence on R5 and R8 also appeared to be linked but these may be identical R-factors. Contrary to a previous suggestion, the locus determining virulence to R10 was independent of that for R5/R8. The expression of virulence to three R-factors (Rl, R4 and R5/R8) was influenced by independent second loci. The presence of a dominant allele at the second locus inhibited avirulence. The expression of avirulence on R6 seemed to be influenced by modifier genes and environment in some isolates. Although the cultivars Mildura, Bourguignonne, Sucrine and Captain were originally thought to contain a single resistance factor, these data suggest that Sucrine carries R5/R8 in addition to R10 whilst Mildura may carry Rl in addition to R3. The stock of Bourguignonne appeared to be a mixture of resistance genotypes. The data add additional support to the suggestion that Capitan (Rll) may carry two resistance factors.     

Variation in the early development of Bremia lactucae on lettuce cultivars with different levels of field resistance

The early stages of development of Bremia lactucae (lettuce downy mildew) were examined on lettuce cultivars possessing high (Iceberg and Regina di Maggio) and low (Great Lakes and Plenos) levels of field resistance. Germ tubes, appressoria, penetration, primary and secondary vesicles, intercellular hyphae and haustoria were observed 3. 6 and 24 h after inoculation of cotyledons and of leaf discs from adult plants. Differences were observed between cv. Iceberg and susceptible genotypes in the percentage of spores germinating and the incidence and speed of development of infection structures. Secondary vesicles were first observed 24 h and 6 h after inoculation in Iceberg and susceptible genotypes, respectively. The lowest incidence of secondary vesicle formation 24 h after inoculation (48 and/or 43%) was recorded in Iceberg and Regina di Maggio, and the highest incidence (68%) occurred in Plenos. The formation of intercellular hyphae and haustoria was not observed in cv. Iceberg some 24 h after inoculation. There were significant differences in the lengths of germ tubes formed on different cultivars. Those on cv. Iceberg were longer than those formed on susceptible cultivars. The results indicate that the field resistance of B. lactucae may result from mechanisms which are effective in the early stages of infection.     

Occurrence in Italy and characteristics of lettuce downy mildew (Bremia lactucae) resistant to phenylamide fungicides

Control of lettuce downy mildew (Bremia lactucae) with phenylamide fungicides has failed in some intensive lettuce-producing areas in Northern Italy since Spring 1993. Before then, these chemicals and particularly metalaxyl, provided the best disease control. The sensitivity of Bremia lactucae isolates collected in such areas to metalaxyl was evaluated in the laboratory. These strains grew and sporulated profusely on lettuce seedlings treated with 100 and 200 ppm of metalaxyl, whereas sensitive control strains were completely inhibited when treated with fungicide concentrations ranging from 0.5–1 ppm. Thus in Italy occurrence of resistance to phenylamide fungicides in Bremia lactucae has also been demonstrated, as in almost all the countries where these chemicals were previously authorised. Subsequently, a demonstration of occurrence of resistance was made and the virulence pattern of several strains (resistant and sensitive to metalaxyl) was characterised using differential NL series containing the 13 DM resistant genes or R factors. The results suggest the occurrence of a new pathotype in Italy different from all the 16 NL Bremia lactucae races studied.     

Virulence characteristics of Bremia lactucae populations in Norway

Use of resistant cultivars represent an efficient control measure for lettuce downy mildew (Bremia lactucae), although the durability of presently deployed resistance genes remains uncertain. Our objective was to document the pathogenic diversity of B. lactucae isolates in Norway. A total of 69 isolates of B. lactucae were collected between 2001 and 2006 from 65 commercial fields and four greenhouses in southeastern and southwestern Norway and tested for the presence of one or more of 19 virulence factors (v-factors). Phenotypic diversity was calculated based on presence or absence of v-factors, and as an overall comparison of v-phenotypes for each isolate. Disease severity varied over the years of the study, and epidemics were most consistently severe in southeastern Norway. The most commonly occurring v-factors, in order of frequency, were v5/8, v7, v2, v18, v4, v13, v6, v11, v12, v1 and v10. Virulence factor v17 was not found, while v36 was found in one isolate only. A total of 44 different v-phenotypes were identified within the population represented by the 69 isolates, yielding an incidence of unique virulence types of 63 %; a relatively high level of pathogen diversity. Four of the identified v-phenotypes were identical to races Bl:17, 18, 22 and 24, which have been previously reported in European populations of B. lactucae. The variability of the Norwegian B. lactucae populations verifies the genetic flexibility of this pathogen and its great ability to adapt to changes in host plants and surrounding conditions.     

A new race of Fusarium oxysporum f. sp. lactucae of lettuce

Fusarium oxysporum f. sp. lactucae, the causal agent of fusarium wilt of lettuce (Lactuca sativa), occurs in most countries in which lettuce is grown and causes serious economic losses. Three races (1, 2 and 3) of the pathogen have previously been identified on the basis of their ability to cause disease on differential lettuce cultivars, as well as by means of molecular tools developed to characterize different races of this pathogen. Only race 1 has been detected in Europe so far. In this study, two isolates of F. oxysporum, obtained from lettuce plants grown in the Netherlands showing symptoms of wilt, have been characterized by combining the study of pathogenicity with differential cultivars of lettuce and molecular assays to determine whether the isolates are different from the known races of F. oxysporum f. sp. lactucae. This study reports the presence of F. oxysporum f. sp. lactucae for the first time in the Netherlands. The causal pathogen has been identified, using the IRAP‐SCAR technique, as a new race of F. oxysporum f. sp. lactucae. Specific primers have been designed to identify this new race.     

Sexual compatibility types of Bremia lactucae isolates originating from Lactuca serriola

Results are given on the occurrence of sexual compatibility types of seven isolates ofBremia lactucae originating fromLactuca serriola (prickly lettuce). It is concluded that the isolates studied are heterothallic. Both compatibility types (B1 en B2) were determined, but type B2 was prevalent. Sexual recombination ofB. lactucae isolates originating from wild and cultivated lettuce may occur.Samenvatting Zeven isolaten vanBremia lactucae, afkomstig vanLactuca serriola in Tsjechoslowakije, zijn onderzocht op hun sexuele compatibiliteitstype door ze te combineren met Nederlandse fysio's vanB. lactucae, afkomstig van cultuursla (L. sativa), waarvan het compatibiliteitstype (B1 of B2) bekend is. Alle isolaten vanL. serriola bleken heterothallisch te zijn, waarbij type B2 meer werd aangetroffen dan type B1. Sexuele recombinatie vanBremia-isolaten van wildeLactuca-soorten en cultuursla blijkt goed mogelijk te zijn.     

Genetic analysis of factors for resistance to downy mildew (Bremia lactucae) in species of lettuce (Lactuca sativa and L. serriola)

Simultaneous genetic analysis of resistance in species of lettuce (Lactuca sativa and L. serriola) and a virulence in Bremia lactucae were made to characterize the genetic basis of resistance to downy mildew in 11 host cultivars and accessions. Four new downy mildew resistance genes (Dm13, Dm14, Dm15 and Dm16) were characterized. Other resistance phenotypes were conditioned by combinations of these or previously described Dm genes. New resistance genes, Dm 16 (from LSE/18), Dm14 (from Gelber Winterkonig) and Dm15 (from PIVT 1309), mapped to Dm linkage group I (Dm1, Dm2, Dm3 and Dm6) while Dm13 from Pennlake segregated independently of the three previously characterized linkage groups and therefore belongs to a fourth linkage group. Characterization of new resistance genes requires simultaneous consideration of host and pathogen genetics.     

The nature of virulence in isolates of Bremia lactucae from South Devon

Nine isolates of Bremia lactucae from South Devon showed the presence of virulence factors against all 11 resistance factors available in lettuce, but no insensitivity to metalaxyl. The virulence phenotype of a single–sporangium isolate of the fungus changed from VI, 3, 4 and 7 to 1, 2, 3, 4, 6 and 7 after it was maintained on Lobjoit Green Cos (RO) for 10 months.     

The occurrence, characteristics and distribution in the United Kingdom of resistance to phenylamide fungicides in Bremia lactucae (lettuce downy mildew)

Failure to control Bremia lactucae (lettuce downy mildew) with metalaxyl in an intensive lettuce-producing region of Lancashire at the end of 1983 was shown to be due to the occurrence of a high level of resistance to this fungicide (isolates capable of growth at < 100 μg/ml metalaxyl). During most of 1984, metalaxyl-resistant isolates were obtained from numerous sites but all within a 20-km radius of the initial outbreak. Thereafter, at the end of 1984 and during 1985, metalaxyl-resistant isolates were recovered from most major lettuce-producing regions in the UK with protected crops more affected than field crops. AH metalaxyl-resistant isolates tested were identical in their response to fungicide, sexual compatibility type (B2) and virulence phenotype, probably representing a clone from a single origin. The resistant pathotype was virulent on resistance factors R 1-10 and 12-15 but lacked virulence for R 11 and 16-18. This was also the most common virulence phenotype among sensitive isolates collected at the same time. Cross-resistance to other phenylamide fungicides was demonstrated but isolates resistant and sensitive to phenylamide showed a similar response to the unrelated systemic fungicides propamocarb and fosetyl-Al. An F₁ sexual progeny isolate from a cross between a phenylamide-sensitive and a phenylamide-resistant isolate (presumed heterozygous at the locus or loci regulating response to phenylamide fungicides) exhibited an intermediate response to phenylamide fungicides. No isolates of this type were obtained from the field. At the high concentrations affecting spore germination, phenylamide fungicides exhibited lower activity against a resistant isolate compared with a sensitive isolate. The findings are discussed in relation to future control strategies, the population biology of the fungus and possible directions for lettuce breeding programmes.     

Activity of carboxylic acid amide (CAA) fungicides against Bremia lactucae

Four carboxylic acid amide (CAA) fungicides, mandipropamid (MPD), dimethomorph (DMM), benthiavalicarb (BENT) and iprovalicarb (IPRO) were examined for their effects on various developmental stages of Bremia lactucae, the causal agent of downy mildew in lettuce, in vitro and in planta. Spore germination in vitro or on leaf surfaces was inhibited by all CAA fungicides (technical or formulated). MPD was more effective in suppressing germination than DMM or BENT, whereas IPRO was least effective. CAA induced no disruption of F-actin microfilament organisation in germinating spores of B. lactucae. CAA applied to germinating spores in vitro prevented further extension of the germ tubes. When applied to germinated spores on the leaf surface they prevented penetration. Preventive application of CAA to intact plants inhibited infection. MPD was more effective in suppressing infection than DMM or BENT, whereas IPRO was least effective. Curative application was effective at ≤3 h post-inoculation (hpi) but not at ≥18 hpi. CAA (except IPRO) applied to upper leaf surfaces inhibited spore germination on the lower surface and hence reduced infection. CAA suppressed sporulation of B. lactucae on floating leaf discs and when sprayed onto infected plants two days before onset of sporulation. BENT and DMM were more effective in suppressing sporulation than MPD or IPRO. Epidemics of downy mildew in shade-house grown lettuce were suppressed by CAA. A single spray applied to five-leaf plants before transplanting controlled the disease for 50 days. The results suggest that CAA are effective inhibitors of spore germination and therefore should be used as preventive agents against downy mildew of lettuce caused by B. lactucae.     

Factors Affecting the Survival of Bremia lactucae Sporangia Deposited on Lettuce Leaves

ABSTRACT Experiments to identify the factors affecting survival of Bremia lactucae sporangia after deposition on lettuce leaves were conducted in growth chambers and outdoors under ambient conditions. Lettuce seedlings at the four-leaf stage were inoculated with B. lactucae sporangia under dry conditions. Sporangia deposited on lettuce seedlings were incubated at different temperature and relative humidity (RH) combinations, exposed to 100, 50, 25, and 0% sunlight in the second experiment, and exposed to different artificial lights in wavelength ranges of UVA (315 to 400 nm), UVB (280 to 315 nm), or fluorescent light in the third experiment. After exposure for 0 to 48 h in the first experiment and 0 to 12 h in the second and third experiments, seedlings in two pots were sampled for each treatment, and sporangia were washed from 15 leaves excised from the sampled seedlings. Germination of sporangia was determined in water after incubation in the dark at 15 degrees C for 24 h. The sampled seedlings with remaining leaves were first transferred to optimal conditions for infection (24 h), for the development of downy mildew, and then assessed for disease after 9 days. Sporangia survived much longer at 23 degrees C (>12 h) than at 31 degrees C (2 to 5 h), regardless of RH (33 to 76%). Germination percentage was significantly reduced after exposure to 50 and 100% sunlight. UVB significantly reduced sporangium viability, while fluorescent light and UVA had no effect relative to incubation in the dark. Infection of seedlings followed a pattern similar to germination of sporangia. Solar radiation is the dominant factor determining survival of B. lactucae sporangia, while temperature and RH have small, insignificant effects in coastal areas of California. This suggests that infections by sporangia that survived a day are probable only on cloudy days or on leaves that are highly shaded.     

Impact of Diurnal Periodicity, Temperature, and Light on Sporulation of Bremia lactucae

ABSTRACT We evaluated direct and interactive effects of light quality and intensity, temperature and light, diurnal rhythms, and timing of high relative humidity during long day lengths on sporulation of Bremia lactucae, the causal agent of lettuce downy mildew, using inoculated lettuce seedlings and detached cotyledons. Suppression of sporulation by light was strongly dependent upon temperature and there was little suppression at 相似文献